To carry out isotopic ratio analysis, high-precision mass spectrometers (IRMS) are used. They are fed with gaseous substances. Special considerations therefore have to be taken into account when analyzing solids or liquids. These may, for example, be provided as a mixture via a liquid chromatograph (LC). In the liquid chromatograph, the individual constituents of the liquid are separated with respect to time. One use of liquid chromatography is for substances which contain carbon, nitrogen and/or sulphur. For isotopic ratio determination of the substances mentioned, conversion to a gas is necessary. Suitable gases are typically CO2, N2 and SO2.
Conventional processes for coupling liquid chromatography with an isotope mass spectrometer are based on substantially removing the eluent or solvent before the substance to be analyzed is converted to a gas. An example of such a process is the “moving wire” principle. In this process, the eluate leaving the liquid chromatograph is evaporated on a continuously moving wire and the remaining dry analysis substance is subsequently converted to a gas in an incineration reactor, see W. A. Brandt and P. Dobberstein: Isotope ratio monitoring liquid chromatography mass spectrometry (IRM-LCMS): First results from a moving wire interface system, Isotopes ENVIRON. HEALTH STUD., 1996, Vol. 32, 275–283.
A further process which has been used for coupling a liquid chromatograph to an isotope mass spectrometer is based on a desolvation of the eluate at semipermeable membranes and subsequent chemical reaction of the dry aerosol generated. (Continuous-Flow Isotope Ratio Mass Spectrometry Using the Chemical Reaction Interface with Either Gas or Liquid Chromatography Introduction, Yohannes Teffera, Josef J. Kusmierz and Fred P. Abramson, Anal. Chem., 1996, 68, 188–1894).